I've got the book on my pile for ages now (just waiting for the perfect reason to pick it up -- seems I've got my wish come true) and my question is, is the film really that bad according to the general consensus at the time?

I think it's worth watching and not nearly as bad as people say it is, but be warned that Candice Bergen's performance is awful. Anyway, other then that it's a pretty decent movie. Nothing earth-shattering, but I can think of much worse ways to spend an hour and forty minutes. Though I will admit that I gave up trying to figure out what was going on around the end of the movie (never read the book).
I just hope Fox properly restores it, or at least uses a decent print. The bootleg I bought off eBay is awful: cropped, full of damage, and it's full of those "VHS lines" despite being a DVD(-R).

I went into this knowing only that Anthony Quinn plays some kind of magician on a Greek island and that everyone hates the movie. Neither of those things is quite true now that I've seen it, as I quite liked this unusual film. While its constant upturnings of the narrative are often ludicrous, I enjoyed never knowing what would happen next (other than that it would somehow retcon all that came before it) and it seems unfair that this film gets ragged on while the not dissimilar and far less bold

SpoilerShow

the Game

gets lavish (over)praise and a Criterion Collection release. As for Candice Bergen, well, she was still in her period of only being called upon to look pretty (and was probably not yet capable of more if she tried), but it's nowhere near a redux of her atrocious Sand Pebbles work two years prior-- and it's not like anyone ever in the history of cinema could have done a better job with the role she's given here! Kudos to Signal One for putting this out on Blu-ray and raising its profile-- the film is postmodern enough to have cycled through being embarrassing on first release and arrive back at "interesting" now.

I bought the Signal One set as a result of Domino's recommendation and I have to agree that, while the film is obviously not a masterpiece, its reputation as a disaster is completely undeserved: I thought it was quite a brave attempt to adapt a difficult book which clearly wished to respect the intelligence of its audience.

The film also made me regret even more the fact that John Boorman was never able to make his proposed adaptation of Lindsay Clarke's The Chymical Wedding, a book which is clearly influenced by The Magus.